Anomalous low-energy phonons in nearly tetragonal BiFeO3 thin films

Abstract

We present evidence for a concomitant structural and ferroelectric transformation around T-S similar to 360 K in multiferroic BiFeO3/LaAlO3 thin films close to the tetragonal phase. Phonon excitations are investigated by using Raman scattering as a function of temperature. The low-energy phonon modes at 180-260 cm(-1) related to the FeO6 octahedron tilting show anomalous behaviors upon cooling through T-S: (i) a large hardening amounting to 15 cm(-1), (ii) an increase of intensity by one order of magnitude, and (iii) an appearance of a dozen new modes. In contrast, the high-frequency modes exhibit only weak anomalies. This suggests an intimate coupling of octahedron tilting to ferroelectricity leading to a simultaneous change of structural and ferroelectric properties.